Photoelectric intrusion detector

ABSTRACT

A photoelectric intrusion detector for detecting the interruption by an intruder of a monitoring beam of optical radiation such as an infrared radiation includes circuitry for preventing the generation of a false alarm when the optical beam is attenuated during its propagation through space due to fog or the like. This false alarm preventing circuitry is designed so that when the attenuation of a beam of optical radiation during its propagation through space is increased due to the occurrence of fog in cloudy weather, the occurrence of a false alarm due to the attenuation of the pulsed light in cloudy weather is prevented by decreasing a comparator reference value to follow the decrease in the level of a light receiving signal, correcting the gain of AGC amplification to maintain constant the level of a received light signal or correcting the level of a received light signal which follows a received light initial value.

BACKGROUND OF THE INVENTION

The present invention relates to a photoelectric intrusion detectorwhich detects interruption of a beam of optical radiation, e.g.,infrared radiation, by an intruder to generate an alarm output.

Systems for detecting interruption of a beam of infrared radiation togive an alarm indicating the presence of an intruder are known in theart as shown in U.S. Pat. No. 3,752,978 to W. G. Kahl, Jr, et al or U.S.Pat. No. 4,516,115 to R. A. Frigon et al.

However, the conventional intrusion detectors have been disadvantageousin that while the outdoor use of the detector in a clear air conditionsuch as fine weather does not greatly attenuate the arriving pulsedlight from a light emitting unit during its propagation through space,thus ensuring a received light level of a sufficient light intensity ata light receiving unit, the occurrence of fog, rainfall or the likeincreases the attenuation of the arriving pulsed light during itspropagation through space so that the received light level is decreasedand the resulting input voltage to a level comparator within thereceiving-end unit becomes lower than a predetermined reference voltage,thereby giving rise to the danger of issuing a false alarm.

Although the occurrence of such a false alarm due to fog or rainfall incloudy weather can be prevented by increasing the intensity of thepulsed light from the light emitting unit, there is of course alimitation to the light emission power of an infrared light emittingdiode of the type generally used in the light emitting unit. Therefore,if the established warning distance between the transmitting andreceiving units is increased, the attenuation of the pulsed lightreaches to a level which cannot be ignored, thereby tending to cause afalse alarm in cloudy weather and thus there is a restriction that theestablished warning distance cannot be increased considerably.

SUMMARY OF THE INVENTION

The present invention has been made in view of the foregoingdeficiencies in the prior art and it is the primary object of theinvention to provide a photoelectric intrusion detector which isdesigned to prevent as far as possible the occurrence of any false alarmdue to the attenuation of a beam of optical radiation during itspropagation through space in cloudy weather accompanied with theoccurrence of fog or the like.

To accomplish the above object, in accordance with one aspect of theinvention there is provided a photoelectric intrusion detector includinglight sensitive means which receives the pulsed light projected from aseparately arranged light emitting unit at a place remote therefrom togenerate a corresponding electric signal; first comparison means forcomparing the electric signal with a separately applied referencevoltage to remove from the electric signal the noise component which islower than the reference voltage; circuit means for receiving an outputfrom the first comparison means to generate an output signal having a DClevel corresponding to the received output; second comparison means forcomparing the DC level signal generated from the circuit means with apredetermined threshold value to generate an output when the DC level islower than the threshold value; alarm signal generating means forgenerating an alarm output when the output of the second comparisonmeans lasts over a predetermined storage time; and reference voltagegenerating means for producing a voltage output varying in response tovariations in the output of the light sensitive means with apredetermined delay time exceeding the storage time of the alarm signalgenerating means so as to apply said voltage output to the firstcomparison means as said reference voltage.

When fog occurs in cloudy weather, the received light signal level isattenuated and also the level of noise included in the received lightsignal is attenuated. With the photoelectric intrusion detector of theinvention constructed as above described, the reference voltage of thefirst comparison means for removing the noise component is decreased inresponse to decrease in the received light signal level with apredetermined delay time. When this occurs, even if the received lightsignal is decreased due to fog or rain, the comparison voltage level forremoving the noise component is decreased correspondingly so that in thesteady-state monitoring condition the received light pulse (comparatoroutput) from which the noise has been positively removed is obtainedstably. Therefore, even if the attenuation of the pulsed light isincreased in cloudy weather, there is no danger of causing any falsealarm.

On the other hand, the interruption of the beam of infrared radiation bythe passage of an intruder is effected at a higher speed than decreasein the light receiving signal level due to fog or rain and theinterruption is positively detected in accordance with the difference inrate of decreasing change between the two, thereby leading to thegeneration of an alarm signal. In other words, since the referencevoltage decreases to follow the signal level after the time delay longerthan the storage time of the alarm signal generating means, in responseto a rapid decrease in the light receiving level due to the passage ofan intruder, the second comparison means immediately generates an outputand the duration time of this output reaches a time sufficient for thealarm signal generating means to generate an alarm signal before thereference voltage decreases due to the decrease in the light receivinglevel, thereby positively giving an alarm.

Also, in accordance with another aspect of the invention there isprovided a photoelectric intrusion detector including light sensitivemeans which receives the pulsed light projected from a separatelyarranged light emitting unit at a place remote therefrom to generate acorresponding electric signal; circuit means for receiving the electricsignal generated from the light sensitive means to generate an outputsignal having a DC level corresponding to the electric signal;comparison means for comparing the DC level signal generated from thecircuit means with a separately applied reference voltage to generate anoutput when the DC level is lower than the reference voltage; alarmsignal generating means for generating an alarm output when the outputof the comparison means lasts over a predetermined storage time; andreference voltage generating means for producing a voltage outputvarying in response to variations in the output of the light sensitivemeans with a predetermined delay time exceeding the storage time of thealarm signal generating means so as to apply said voltage output to thecomparison means as said reference voltage. In the photoelectricintrusion detector of the invention having this construction, thereference voltage of the comparison means for performing levelcomparison of the received light signal level itself is decreased inresponse to decrease in the received light signal level with apredetermined delay time.

Further, in accordance with still another aspect of the invention thereis provided a photoelectric intrusion detector including light sensitivemeans which receives the pulsed light projected from a separatelyarranged light emitting unit at a place remote therefrom to generate acorresponding electric signal; automatic gain control amplifying meansfor amplifying the electric signal generated from the light sensitivemeans with a gain corresponding to a separately applied AGC controlvoltage; circuit means for receiving an amplified output of theautomatic gain control amplifying means to generate an output signalhaving a DC level corresponding to the amplified output; comparisonmeans for comparing the DC level signal generated from the circuit meanswith a reference voltage to generate an output when the DC level islower than the reference voltage; alarm signal generating means forgenerating an alarm output when the output of the comparison means lastsover a predetermined storage time; and delay circuit means for supplyingthe DC level signal from the circuit means as the AGC control signal tothe automatic gain control amplifying means with a predetermined delaytime exceeding the storage time of the alarm signal generating means,whereby the automatic gain control is performed in such a manner thatthe input to the comparison means is maintained at a constant level witha given time delay with respect to variations in the light receivingsignal level within the range of automatic gain control where thereceived light signal level of the light sensitive means is higher thana given level.

In the case of the photoelectric intrusion detector constructed asdescribed above, when the received light signal level is decreased, thereceived light signal level is maintained constant by the AGCamplification with a given time delay so that even if the attenuation ofthe pulsed light is increased in cloudy weather, this takes the form ofa relatively slowly varying decrease and no false alarm is caused. Onthe other hand, since the AGC control of the received light signal levelis performed by introducing a delay time exceeding the storage time ofthe alarm signal generating means, when the beam is interrupted by thepassage of an intruder, an alarm signal is positively generated beforethe AGC control voltage is corrected.

Further, in accordance with still another aspect of the invention thereis provided a photoelectric intrusion detector including light sensitivemeans which receives the pulsed light projected from a separatelyarranged light emitting unit at a place remote therefrom to generate acorresponding electric signal; circuit means for receiving the electricsignal generated from the light sensitive means to generate an outputsignal having a DC level corresponding to the electric signal; memorymeans for storing as an initial value the DC level signal generated fromthe circuit means at the time of connection to a power source;correction factor alteration means for comparing a value of the DC levelsignal generated from the circuit means with the initial value held inthe memory means at a predetermined constant period so that when thereis a difference between the two values, a correction factor outputvarying in accordance with the difference is generated; operation meansfor performing an operation on the correction factor output from thecorrection factor alteration means and the DC level signal generatedfrom the circuit means to correct the DC level signal to follow thestored initial value with a given time delay; comparison means forcomparing the corrected DC level signal generated from the operatormeans with a predetermined reference voltage to generate an output whenthe DC level of the corrected DC level signal is lower than thereference voltage; and alarm signal generating means for generating analarm signal when the output of the comparison means lasts over apredetermined storage time shorter than the constant period.

In this case, due to the fact that the received light signal level isperiodically corrected to follow the initial value of the received lightsignal level at the time of connection to the power source, even if theattenuation of the pulsed light increases in cloudy weather, this is arelatively slowly varying decrease and therefore no false alarm isgenerated. Also, as regards the correction of the received light signallevel, the correction is effected after the expiration of the delay timeexceeding the storage time of the alarm signal generating means so thatwhen the pulsed light is interrupted by the passage of a person, analarm is issued positively prior to the correction of the received lightsignal level.

Thus, in accordance with the invention, in view of the fact that whenthe attenuation of a pulsed light during its propagation through spaceis increased due to the occurrence of fog or the like in cloudy weather,the occurrence of any false alarm due to the attenuation of the pulsedlight in cloudy weather is prevented positively by virtue of decrease inthe comparator reference value in response to decrease in the receivedlight signal level, correction of the gain of AGC amplification formaintaining the received light signal level constant or correction ofthe received light signal level to follow the received light initialvalue.

The above and other objects and advantages of the invention will be morereadily understood from the following description of its preferredembodiments taken in conjunction with the accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a circuit block diagram showing an embodiment of the presentinvention.

FIGS. 2(a)--2(g) show a plurality of signal waveforms useful forexplaining the operation of the embodiment of FIG. 1.

FIG. 3 is a graph showing the relation in time between the receivedlight output and the reference voltage in the embodiment of FIG. 1.

FIG. 4 is a block diagram showing a second embodiment of the invention.

FIG. 5 is a block diagram showing a third embodiment of the invention.

FIG. 6 is a diagram for explaining the AGC characteristic of theembodiment shown in FIG. 5.

FIG. 7 is a block diagram showing a fourth embodiment of the invention.

FIG. 8 is a diagram showing the correction of the light receiving outputin the embodiment of FIG. 7.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIG. 1 showing a circuit block diagram for a firstembodiment of the invention, numeral 101 designates a light emittingunit, and 102 a light receiving unit. After their optical axes have beenaligned, the two units are separately arranged apart by a given warningdistance.

The light emitting unit 101 includes an oscillator circuit 103 thatoutput light emission drive pulses to an infrared light emitting element104 comprising, for example, a light emitting diode, so as tointermittently drive it for light emission and the resulting lightemission pulses from the infrared light emitting element 104 arecollimated into a collimated beam of light by means of a lens (or acondensing mirror) 105, thereby projecting it to the light receivingunit 102. In this case, the frequency of the pulsed light from the lightemitting unit 101 is generally selected to the about 500 Hz so as toavoid the effect of any disturbing noise light due to a fluorescent lampor the like.

The light receiving unit 102 includes a lens 106 and a light sensitiveelement (photoelectric conversion element) 107 forming a light receivingsection, so that the pulsed light emitted from the light emitting unit101 is condensed through the lens 106 onto the light sensitive element107 which in turn converts it to an electric signal. Of course, acondensing mirror may be used in place of the lens 106.

The weak received light signal generated from the light sensitiveelement 107 is amplified by an amplifier circuit 108 and applied to afir$t level comparator 109. A comparator reference voltage for the firstcomparator 109 is applied from a reference voltage generating circuit116.

The reference voltage generating circuit 116 includes a rectifiercircuit 117, a smoothing circuit 118 and a reference voltage settingadjuster 119. More specifically, the light receiving signal amplified bythe amplifier circuit 108 is rectified by the rectifier circuit 117 andconverted to a DC signal corresponding to the level of the amplifiedlight receiving signal by the smoothing circuit 118 having a given timeconstant (delay time T₁). The smoothed output from the smoothing circuit118 is divided with, for example, a suitable voltage dividing ratio bythe reference voltage setting adjuster 119, thereby applying theresulting voltage as the reference voltage to the comparator 109. As aresult, the reference voltage applied to the comparator 109 from thereference voltage generating circuit 116 is varied in correspondence tothe output level of the amplifier circuit 108.

The noise component of a signal level lower than the reference voltageis removed from the output signal of the amplifier circuit 108 so thatthe received light signal is generated as a noiseless pulse signal fromthe comparator 109. The output pulses from the comparator 109 aresmoothed out by another smoothing circuit 111 so that the output pulsesare converted to a DC level signal and applied to a second comparator112. A second reference voltage serving as a given threshold value isfixedly set for the second comparator 112 by another reference voltagegenerating circuit 113 so that when the output level of the smoothingcircuit 111 becomes lower than the second reference voltage, thecomparator 112 generates an output. The output of the comparator 112 isapplied to a switching circuit 114. When the output of the comparator112 is applied continuously over a given storage time T₂ preset for thepurpose of preventing any false alarm due to a passing small animal or aleaf falling from a tree, the switching circuit 114 comes into operationso that an alarm signal is sent through an output circuit 115 to analarm receiving board or the like (not shown), thereby giving an alarm.

It is to be noted that the value of the time constant (delay time T₁) ofthe smoothing circuit 118 included in the reference voltage generatingcircuit 116, is selected longer than the storage time T₂ of theswitching circuit 114 so that when the pulsed light transmitted from thelight emitting unit 101 to the light receiving unit 102 is interruptedby an intruder, the response or follow-up delay time of the referencevoltage with respect to decrease in the output level of the amplifiercircuit 108 is sufficiently long and therefore the interruption of thepulsed light by the intruder can be detected positively.

Next, the operation of the embodiment of FIG. 1 will be described withreference to the signal waveforms shown in FIGS. 2(a)-2(g). The signalwaveforms of FIGS. 2(a)-2(g) are classified so that the left halfcorresponds to fine weather wherein the attenuation of pulsed light isreduced and the right half corresponds to cloudy weather wherein theattenuation of pulsed light is increased due to the occurrence of fog orthe like.

Firstly, when the weather is fine so that the attenuation of the pulsedlight from the light emitting unit 101 during its propagation throughspace is decreased, the light sensitive element 107 generates a receivedlight signal b of a sufficient level so that this received light signalb is amplified by the amplifier circuit 108 and then applied to thecomparator 109. Also, the amplified output c of the amplifier circuit108 is applied to the reference voltage generating circuit 116 so that arectified and smoothed output d corresponding to the output c of theamplifier circuit 108 is generated by the rectifier circuit 117 and thesmoothing circuit 118 and the rectified and smoothed output d issubjected to voltage division by a given ratio by the reference voltagesetting adjuster 119, thereby generating a reference voltage e for thecomparator 109.

As a result, the comparator 109 removes the noise component contained inthe signal component of the output c of the amplifier circuit 108 whichis lower than the voltage level V_(th1) of the reference voltage e andsimultaneously it generates a comparator output f reshaped intorectangular pulses corresponding to the signal component greater thanthe reference voltage level V_(th1).

The output (pulse output) of the comparator 109 is smoothed by thesmoothing circuit 111 and the resulting smoothed output g is comparedwith the threshold voltage V_(th) from the reference voltage generatingcircuit 113 by the comparator 112. When the smoothed output g is greaterthan the threshold value V_(th), the comparator 112 generates no outputand this condition is a steady-state monitoring condition, thus givingno alarm. On the other hand, when the pulsed light from the lightemitting unit 101 to the light receiving unit 102 is interrupted by thepassage of an intruder, the smoothed output g of the smoothing circuit111 becomes lower than the threshold voltage V_(th) from the referencevoltage generating circuit 113 and the comparator 112 generates anoutput. When the output of the comparator 112 lasts over the given timeT₂, the switching circuit 114 comes into operation so that the outputcircuit 115 transmits an alarm signal to a burglary signal receivingboard which in turn gives a burglary alarm.

Next, the operation of the detector in cloudy weather with theoccurrence of fog or the like will be described.

When the pulsed light from the light emitting unit 101 is attenuated,during its propagation through space due to the occurrence of fog incloudy weather, the output level of a light receiving signal b from thelight sensitive element 107 is decreased and the signal level of anoutput c from the amplifier circuit 108 is also decreased considerablyas compared with that in fine weather.

On the other hand, the noise component contained in the output c of theamplifier circuit 108 is also decreased as the result of reduction inexternal noise light caused by the sunlight, fluorescent lamp or thelike due to the occurrence of fog and at the same time the circuit noisecaused within the circuitry of the light receiving unit 102 is alsorelatively decreased by a decrease in the ambient temperature due to thecloudy weather. Therefore, due to the reduction in the level of theoutput c of the amplifier circuit 108 caused by the attenuation of thepulsed light during its propagation through space, the noise included inthe output c is also decreased in like manner.

This output of the amplifier circuit 108 is applied not only to thecomparator 109 but also to the reference voltage generating circuit 116so that it is applied to the reference voltage setting adjuster 119 as arectified and smoothed output d corresponding to the magnitude of theoutput c from the smoothing circuit 118 through the rectifier circuit117. As a result, the reference voltage generated from the referencevoltage setting adjuster 119 follows the reduction in the level of theoutput c from the amplifier circuit 108 and it is applied to thecomparator 109 as a reference voltage V_(th2) which is sufficiently lowas compared with that in fine weather.

Thus, since the comparator reference voltage of the comparator 109 isreduced to V_(th2) in response to the reduced level of the output c ofthe amplifier circuit 108 due to the attenuation of the pulsed light incloudy weather, the comparator 109 undergoes setting adjustment to theproper reference voltage V_(th2) with respect to the output c of theamplifier circuit 108 corresponding to the received light signal b ofthe reduced level so that the noise component included in the signalcomponent lower than the reference voltage V_(th2) is removed from theoutput c of the amplifier circuit 108 and simultaneously a rectangularpulse signal corresponding to the signal component greater than thereference voltage V_(th2) is generated as a comparator output f. Thus,the smoothed output g generated from the smoothing circuit 111 smoothingthe output f of the comparator 109 is substantially the same in thesteady-state monitoring condition as that in fine weather so that thesmoothed output g never drops below the threshold voltage V_(th) fromthe reference voltage generating circuit 113 and the generation of anyfalse alarm due to the occurrence of fog or the like can be preventedpositively.

It is to be noted that when the reference voltage for the comparator 109is varied in response to the output c of the amplifier circuit 108 bythe reference voltage generating circuit 116, if the reference voltageis caused to decrease immediately in response to the output c upon theinterruption of the pulsed light by an intruder, there is thepossibility of failing to give an alarm when the noise component in theamplified received light receiving signal c exceeds the reference level.However, in this embodiment the time constant (delay time t₁) of thesmoothing circuit 118 included in the reference voltage generatingcircuit 116 is in the form of a time constant exceeding the durationtime (storage time T₂) of the switching circuit 114, with the resultthat upon the interruption of the pulsed light by an intruder or thelike, the delay of the reference voltage in following the reduction inthe received light signal level is increased sufficiently and thus thereis no danger of failing to detect the interruption of the pulsed lightby the intruder.

FIG. 3 is a diagram showing the relation between the received lightoutput and the reference voltage in the embodiment of FIG. 1 during thetransition from fine weather to the occurrence of fog.

FIG. 3 illustrates the proportional relation between the received lightsignal level and the reference voltage for the comparator 109 so thatif, for example, V_(n1) represents the received light output in fineweather where the transmission factor is substantially 100%, the thencurrent reference voltage from the reference voltage setting adjuster119 is represented by V_(r1). In this condition, if the pulsed light isinterrupted by an intruder, due to a rapid decrease in the receivedlight output V_(n), an alarm output AL₁ is generated after the storagetime T₂ and then the reference voltage V_(r) is decreased to follow upwith a time delay T₁. In this monitoring condition, if, for example, fogoccurs at a certain time t_(i) so that the received light output V_(n)begins to decrease, the reference voltage V_(r) is also decreased asshown in the FIG. 3 in response to the decrease in the received lightoutput V_(n) after the delay time T₁.

Then, when the pulsed light is interrupted by the passage of an intruderso that the received light output level drops to V_(n2) at time t_(n),the reference voltage decreases to V_(r3) at a time which is delayed byT₁ from the time that the received light output drops. Thus, since thedecrease in the reference voltage involves the delay in following up, aconsiderable time is required for the reference voltage to attain thelower level V_(r3) than the decreased received light output V_(n2) sothat when the received light output is V_(n2), the reference voltage isstill at the higher level V_(r2) causing the comparator 109 to generatean output and thus after the expiration of the storage time T₂, an alarmsignal AL₂ according to the interruption of the pulsed light can begenerated positively.

On the other hand, when fog occurs so that the received light output isdecreased gradually as shown in FIG. 3, the reference voltagepractically follows up correspondingly. However, when the fog becomesdenser so that a certain lower-limit received light output level V_(n3)corresponding to the transmission factor of 1%, for example, isattained, the received light output level drops below the correspondingreference voltage V_(r4) and thus an alarm signal AL₃ is generated. Thisis due to the fact that it is impossible to distinguish between thepassage of an intruder and the occurrence of fog.

FIG. 4 is a circuit block diagram showing a light receiving unit used ina second embodiment of the invention. A light receiving unit 202receives the pulsed light comprising an infrared radiation beam from alight projecting unit (not shown) by way of a light sensitive element207 through a condensing lens 206. An amplifier circuit 208 amplifiesthe received light signal from the element 207 and the output of theamplifier circuit 208 is converted to a DC level signal by a smoothingcircuit 211. This DC level signal is applied, on the one hand, to acomparator circuit 212 through a DC amplifier circuit 222 and it isapplied, on the other hand, to a DC delay amplifier circuit 210 in areference voltage generating circuit 216. The DC delay amplifier circuit210 generates a DC output which follows variations in the DC levelsignal from the smoothing circuit 211 with a predetermined delay T₁ andthis DC output is divided with, for example a suitable voltage dividingratio by a reference voltage setting adjuster 219, thereby supplying itas a reference voltage to comparator 212. As a result, the referencevoltage applied to the comparator 212 from the reference voltagegenerating circuit 216 is varied in accordance with the output level ofthe amplifier circuit 208. The comparator 212 compares the referencevoltage applied from the reference voltage generating circuit 216 andthe DC level signal from the DC amplifier circuit 222 to generate anoutput when the output level of the DC amplifier circuit 222 is lowerthan the reference level. The output of the comparator 212 is applied toan alarm circuit 224. The alarm circuit 224 comprises, for example, theswitching circuit 114 and the output circuit 115 shown in FIG. 1, sothat when the output of the comparator 212 lasts over a given storagetime (T₂), a detection signal is sent to an alarm receiving board or thelike and a burglar alarm is delivered. It is to be noted that thestorage time T₂ is determined by the specific time constant preset inthe alarm circuit 224 for the purpose of preventing any false alarm dueto a small animal or a leaf.

Then, the value of the time constant (storage time T₁) of the DC delayamplifier circuit 210 included in the reference voltage generatingcircuit 216 is selected longer than the storage time T₂ of the alarmcircuit 224. Thus, when the pulsed light from the light emitting unit tothe light receiving unit 202 is interrupted by an intruder, the delaytime of the reference voltage in following the decrease in the outputlevel of the amplifier circuit 208 is sufficiently large and thereforethe interruption of the pulsed light by the intruder can be detectedpositively. On the other hand, a relatively slow decrease in the outputlevel due to fog or the like cannot cause any false alarm since thereference voltage of the comparator circuit 212 decreases to follow it.

FIG. 5 shows the principal construction of a light receiving unit usedin a third embodiment of the invention wherein a lens 306, a lightsensitive element 307, a comparator 312 and a reference voltagegenerating circuit 313, which are included in the light receiving unit,respectively correspond to the lens 106, the light sensitive element107, the second comparator 112 and the reference voltage generatingcircuit 113 in the embodiment of FIG. 1 and therefore their detailedexplanation will be omitted. Also, an alarm circuit 324 comprises, forexample, the switching circuit 114 and the output circuit 115 shown inFIG. 1 so that when the output of the comparator 312 continues over agiven storage time (T₂), a detection signal is sent to an alarmreceiving board or the like and an alarm is given.

In addition, in the embodiment of FIG. 5 an AGC amplifying circuit 320,a smoothing circuit 321, a DC amplifier circuit 322 and a delay circuit323 are arranged between the light sensitive element 307 and thecomparator 312.

The AGC amplifying circuit 320 has an automatic gain control function ofamplifying the received light output of the light sensitive element 307and a gain to be controlled is determined by the AGC control voltagegenerated from the delay circuit 323. The smoothing circuit 321 smoothesthe amplified output from the AGC amplifying circuit 320 to convert itto a DC voltage signal. The DC amplifier circuit 322 amplifies the DCvoltage signal generated from the smoothing circuit and applies it tothe comparator 312. Also the output of the DC amplifier circuit 322 isapplied to the delay circuit 323 so that after the introduction of agiven time delay, it is applied as the AGC control voltage to the AGCamplifying circuit 320.

Here, if the storage time of the alarm circuit 324 is represented by T₂,the delay time T₁ of the delay circuit 323 is set to a constant valueexceeding the storage time T₂.

FIG. 6 shows an AGC characteristic of the AGC amplifying circuit 320 inthe embodiment of FIG. 5, with the abscissa representing thetransmission factor (input) of the beam of infrared radiation betweenthe light emitting and receiving units and the ordinate representing theDC output voltage of the DC amplifier circuit 322.

In this example, the AGC characteristic of the AGC amplifying circuit320 is a combined characteristic of A₀ and A₅ of FIG. 6. In other words,a constant DC output voltage determined by the characteristic A₀ isgenerated for the transmission factors of over 10% of the pulsed lightcorresponding to the received light output of the light sensitiveelement 107 and the characteristic A₅ which decreases the DC outputvoltage linearly with decrease in the transmission factor is set for thetransmission factors of less than 10%. In this way, the AGC controlledrange is set to maintain the DC output voltage constant for thetransmission factors of over 10%.

In addition, the characteristic diagram of FIG. 6 also shows the caseswithout AGC, i.e., non-AGC characteristics A₁, A₂, A₃ and A₄ withrespect to the transmission factors of 100%, 75%, 50% and 30% in thesteady-state condition.

Note that in FIG. 6 the dot-and-dash line shows the alarming leveldetermined by the reference voltage V_(r) applied from the referencevoltage generating circuit 313 to the comparator 312.

In FIG. 6, the transmission factor (the alarming transmission factor: analarm signal is generated when decreasing to this transmission factor)and the alarming rate of change (the amount of decrease of thetransmission factor required for the generation of an alarm signal) ateach of the intersection points P₁ to P₅ of the alarming level V_(r) andthe non-AGC characteristics A₁ to A₅ for the transmission factors of100%, 75%, 50%, 30% and 10%, respectively, are shown by way of examplein the following Table 1.

                  TABLE 1                                                         ______________________________________                                        Steady-state   Alarming   Alarming                                            transmission   transmission                                                                             rate of                                             factor         factor     change                                              ______________________________________                                        100%           12% (88%)  88%                                                 75%            9% (91%)   66%                                                 50%            7% (93%)   43%                                                 30%            4% (96%)   26%                                                 10%            1% (99%)    9%                                                 ______________________________________                                         Note that the parentheses of alarming transmission factors indicates the      rates of beam attenuation.                                               

Next, the operation of the embodiment of FIG. 5 will be described withreference to the characteristic diagram of FIG. 6.

Now, in the condition where the transmission factor of the pulsed lightbetween the light emitting unit and the light receiving unit is 100%,due to the AGC function of the AGC amplifying circuit 320, the DC outputvoltage at the intersection point P₀₁ of the AGC characteristic A₀ andthe non-AGC characteristic A₁ for the transmission factor of 100% isgenerated from the DC amplifier circuit 322.

In this condition, if the transmission factor is slowly decreased to 75%due to, for example, the occurrence of fog, the operating point is movedto the intersection point P₀₂ of the AGC characteristic A₀ and thenon-AGC characteristic A₂ for the transmission factor of 75% so thatsince the resulting changed transmission factor is in the AGC controlledrange, the DC output voltage from the DC amplifier circuit 322 ismaintained at the constant voltage according to the same AGCcharacteristic A₀.

In like manner, when the transmission factor between the two units isdecreased successively to 50%, 30% and 10%, respectively, similarly theoperating point is moved successively to points P₀₃, P₀₄ and P₀₅,respectively, so that since these points lie on the same AGCcharacteristic A₀, the DC output voltage from the DC amplifier circuit322 is always maintained constant.

As a result, in the AGC controlled range where the transmission factoris greater than 10%, even if the transmission factor of the pulsed lightis changed by the attenuation due to the occurrence of fog, the DCoutput voltage applied from the DC amplifier circuit 322 to thecomparator 312 is always maintained at the constant level owing to theAGC amplifying function performed by the AGC amplifying circuit 320, sothat when the DC output voltage is compared with the reference voltageof the constant value by the comparator 312, there is no danger ofcausing any false alarm even in fog or the like.

On the other hand, in the condition where the transmission factor is,for example, 100% in FIG. 6, if the pulsed light between the two unitsis interrupted by the passage of an intruder, the delay time T₁ isrequired, due to the delay circuit 323, for variation of the AGC controlvoltage applied to the AGC amplifying circuit 320 and the delay time T₁is selected to be longer than the storage time T₂ of the alarm circuit324. Therefore, prior to the AGC amplifying function of the AGCamplifying circuit 320 becoming effective, the DC output voltage isdecreased in accordance with the non-AGC characteristic A₁ of FIG. 6 sothat when the DC output voltage drops below the point P₁ intersectingthe reference voltage V_(r) preset as the alarming level in thecomparator 312, the comparator 312 generates an output. The output ofthe comparator 312 is generated over the delay time T₁ introduced by thedelay circuit 323 so that since the storage time T₂ of the alarm circuit324 is selected to be shorter than the delay time T₁, prior to thegeneration of the amplified output according to the characteristic A₅owing to the AGC amplification of the AGC amplifying circuit 320, thealarm circuit 324 generates an output and the passage of the intruder isdetected positively, thereby giving an alarm.

The same operation takes place in cases where the transmission factorbetween the units in the steady-state monitoring condition is 75%, 50%,30% or 10%, so that prior to the AGC function of the AGC amplifyingcircuit 320 becoming effective due to the delay time T₁ of the delaycircuit 323, the DC output voltage from the DC amplifier circuit 322 isdecreased in accordance with the non-AGC characteristic A₂, A₃, A₄ orA₅. Thus the comparator 312 generates an output when the DC outputvoltage drops below the point P₂, P₃, P₄ or P₅ intersecting thereference voltage V_(r) establishing the alarming level. Then, when theoutput of the comparator 312 lasts over the storage time T₂ preset inthe alarm circuit 324, an alarm based on the detection of the intruderis generated. Note that while, in this case, it is impossible todetermine whether the alarm is due to the occurrence of fog or thepassage of an intruder at the time of the point P₅, in any event theprimary object is accomplished by the generation of the alarm.

It is to be noted here that the introduction of the delay in the AGCcontrol has the effect that the greater the transmission factor betweenthe units in the steady-state monitoring condition, correspondinglygreater is the alarming transmission factor as shown in Table 1. Forinstance, while, in fine weather, the transmission factor in thesteady-state monitoring condition is high, at the time the level ofnoise due to scattered light or the like is also high so that even ifthe pulsed light or the like is also high so that even if the lightreceiving output is not decreased sufficiently due to the noise. In thiscase, if no delay is introduced in the AGC control, unless the lightreceiving output drops to such an alarming transmission factor (about 1%at the point P₅) that the DC output voltage from the DC amplifiercircuit 322 is decreased down to the reference voltage V_(r) inaccordance with the characteristics A₀ and A₅ of FIG. 6, the comparator312 does not generate an output so that an alarm signal cannot begenerated depending on the noise level and there is the danger offailing to given an alarm. On the contrary, in the present embodimentthe delay is introduced in the AGC control so that the alarmingtransmission factor is shifted in level in accordance with variations ofthe transmission factor between the units. Thus as the transmissionfactor between the units in the steady-state monitoring condition isincreased, the comparator 312 generates an output earlier at a higheralarming transmission factor corresponding to the point P₂ or P₁ in FIG.6.

Note that in this case, if the reference voltage V_(r) were decreased tofollow the light receiving output without any sufficient time delay, infine weather, for example, the noise light other than the pulsed lightis so ample that a received light output of a fair level is present evenif the pulsed light is interrupted and therefore an alarm cannot begenerated, thus failing to give an alarm.

Referring to FIG. 7, there is illustrated a circuit block diagram of alight receiving unit used in a fourth embodiment of the invention.

In FIG. 7, a lens 406, a light sensitive element 407, an amplifiercircuit 408, a smoothing circuit 411, a comparator 412, a referencevoltage generating circuit 413, which are included in the lightreceiving unit, respectively correspond to the lens 106, the lightsensitive element 107, the amplifier circuit 108, the smoothing circuit111, the second comparator 112 and the reference voltage generatingcircuit 113 in the embodiment of FIG. 1 and will not be described in anydetail. Also, an alarm circuit 424 comprises, for example, the switchingcircuit 114 and the output circuit 115 shown in FIG. 1 so that when theoutput of the comparator 412 lasts over a given storage time (T₂), adetection signal is sent to an alarm receiving board or the like and aburglary alarm is issued.

In addition, in the embodiment of FIG. 7 a memory circuit 425, acorrection factor alteration circuit 426, a timer 427 and an operationcircuit 428 are arranged between the smoothing circuit 411 and thecomparator 412.

The memory circuit 425 stores and holds as a received light initialvalue V₀ the received light output generated from the smoothing circuit411 when the detector is connected to a power source. In other words,the memory circuit 425 stores and holds as the received light outputinitial value V₀ the received light output generated in a conditionwhere the transmission factor is 100% with no dirt on the lens, etc. Thecorrection factor alteration circuit 426 compares the stored initialvalue V₀ of the memory circuit 425 and the then current received lightoutput V_(n) generated from the smoothing circuit 411 at a given periodpreset by the timer 427, so that if there is a difference between thetwo, that is, if the then current received light output V_(n) isdecreased by the change of the transmission factor due to the occurrenceof fog, a correction factor alteration function is performed to computea new correction factor K_(n) in accordance with the stored initialvalue V₀ and the then current light receiving output V_(n).

In other words, when there is a difference between the stored initialvalue V₀ and the light receiving output V_(n), the correction factoralteration circuit 426 performs the following calculation so that a newcorrection factor is determined and the previous correction factor isreplaced with the new correction factor

    K.sub.n =V.sub.0 /V.sub.n

On the other hand, the alteration period of the correction factoralteration circuit 426 which is determined by the timer 427 is such thatthe alteration is effected at intervals of a given time T₁ which isgreater than the storage time T₂ of the alarm circuit 424.

In accordance with the then current correction factor K_(n) determinedby the correction factor alteration circuit 426 and the received lightoutput V_(n) generated from the smoothing circuit 411, the operatorcircuit 428 performs the following correcting operation and outputs thecorrected received light output to the comparator 412

    V.sub.a =K.sub.n ×V.sub.n

A circuit section 430, including the memory circuit 425, the correctionfactor alteration circuit 426, the timer 427 and the operator circuit428, has a function of performing a correcting computation such that thereceived light output always follows the received light output initialvalue V₀ stored and held in the memory circuit 425 with a delay of thepreset period by the timer 427 in response to decrease in thetransmission factor. Note that when the level V_(n) of the DC voltageoutput from the smoothing circuit 411 drops belows a limit referencevoltage V_(s) separately applied from the reference voltage generatingcircuit 413, the operation of the timer 427 is stopped by a timer stopcircuit 429. The reason for this is that when the received light outputdrops below the given level, the operation of the correction factoralteration circuit 26 is stopped so that the level of the DC voltageV_(a) applied to the comparator 412 is decreased and at any rate analarm signal is generated, although it is not certain whether thedecrease of the received light level is due to fog or an intruder.

The operation of the embodiment of FIG. 7 will now be described withreference to FIG. 8.

FIG. 8 shows the variation of the corrected received light outputgenerated from the operator circuit 428 in the event that the receivedlight output is decreased linearly due to the occurrence of fog or thelike.

In other words, with the light receiving initial value V₀ being storedand held in the memory circuit 425 upon connection to the power source,the time 427 applies a command for alteration computation to thecorrection factor alteration circuit 426 at the predetermined period T₁so that when there is a difference between the stored initial value V₀and the received light output V_(n), a new correction factor K_(n) iscomputed.

For instance, assuming that the stored initial value V₀ =100 and thereceived light output V₁ =98, the correction factor alteration circuit426 performs the computation of K₁ =V₀ +V₁ =100/98=1.2 and then theoperator circuit 428 performs the computation of a corrected receivedlight output V_(a1) =1.02×98=99.96 by using the correction factor K₁=1.02 of the correction factor alteration circuit 426, therebydetermining the corrected received light output V_(a1) which issubstantially equal to the stored initial value V₀ =100.

Thereafter, each time the predetermined period T₁ expires, the timer 427similarly applies a command for alteration computation to the correctionfactor alteration circuit 426 so that new correction factors K₂, K₃, K₄,. . . , are determined after thereby outputting to the comparator 412corrected light receiving outputs V_(a2), V_(a3), V_(a4), . . . whichare practically equal to the stored initial value V₀.

As a result, even if the received light output is decreased by adecrease in the transmission factor due to the occurrence of fog or thelike, the corrected received light output applied to the comparator 412is maintained at substantially the constant initial value V₀ and thegeneration of a false alarm from the alarm circuit 424 is preventedpositively. However, this cannot generate an alarm even if the pulsedlight is completely interrupted by the fog so that the time stop circuit429 stops the alteration of the correction factor upon the decrease to agiven received light output.

On the other hand, when the pulsed light is interrupted by the passageof a person as indicated at a time t_(n) in FIG. 8, the requiredcorrected received light output V_(an) is applied to the comparator 412by the computation of the operator circuit 428 using the correctionfactor corrected just before the time t_(n). Since this correctedreceived light output V_(an) is the one corrected by the correctionfactor before the interruption of the pulsed light, it decreasesconsiderably below the reference voltage of the comparator 412 and thusthe comparator 412 applies an output to the alarm circuit 424. When thiscomparator output lasts so that it reaches the storage time T₂ of thealarm circuit 424 before reaching the next correction period, the alarmcircuit 424 generates an alarm signal. In this way, even if thecorrection of the received light output is being effected, the passageof a person is positively detected to give an alarm.

It is to be noted that while, in the above-described embodiment, thecorrection factor K_(n) is computed by the correction factor alterationcircuit 426 as follows

    K.sub.n =V.sub.0 /V.sub.n

it be desirable that a limitation is set to the amount of correctioneffected by each correction period so that in the condition where thepulsed light is interrupted by a person, the restoration of the receivedlight output to the stored initial value V₀ by correction is effected insteps. The reason for this is that if the next correction period isreached with the received light output being decreased by theinterruption of the pulsed light due to the passage of an intruder asindicated at the time t_(n) in FIG. 8, there is the danger of thedecreased received light output being altered to the stored initialvalue V₀ before the expiration of the storage time T₂.

What is claimed is:
 1. A photoelectric intrusion detectorcomprising:light sensitive means for receiving a pulsed light projectedfrom a light emitting unit at a place remote from said light emittingunit to generate a corresponding electric signal; first comparison meansfor comparing said electric signal with a reference voltage to remove anoise component lower than said reference voltage from said electricsignal; circuit means for receiving an output from said first comparisonmeans to generate an output signal having a DC level correspondingthereto; second comparison means for comparing the DC level signalgenerated by said circuit means with a predetermined threshold value togenerate an output when said DC level is lower than said threshsoldvalue; alarm signal generating means for generating an alarm output whenthe output of said second comparison means lasts over a predeterminedstorage time; and reference voltage generating means for producing avoltage output varying to follow variations of the output from saidlight sensitive means with a predetermined delay time exceeding thestorage time of said alarm signal generating means so as to apply saidvoltage output to said first comparison means as said reference voltage.2. A photoelectric intrusion detector comprising:light sensitive meansfor receiving a pulsed light projected from a light emitting means at aplace remote from said light emitting means to generate a correspondingelectric signal; circuit means for receiving the electric signalgenerated by said light sensitive means to generate an output having aDC level corresponding thereto; comparison means for comparing the DClevel signal generated by said circuit means with a reference voltage togenerate an output when said DC level is lower than said referencevoltage; alarm signal generating means for generating an alarm outputwhen the output of said comparison means lasts over a predeterminedstorage time; and reference voltage generating means for producing avoltage output varying to follow variations of the output from saidlight sensitive means with a predetermined delay time exceeding thestorage time of said alarm signal generating means so as to apply saidvoltage output to said comparison means as said reference voltage.
 3. Aphotoelectric intrusion detector comprising:light sensitive means forreceiving a pulsed light projected from a light emitting unit at a placeremote from said light emitting unit to generate a correspondingelectric signal; automatic gain control amplifying circuit foramplifying the electric signal generated by said light sensitive meanswith a gain determined by an AGC control voltage; circuit means forreceiving the amplified output from said automatic gain controlamplifying means to generate an output signal having a DC levelcorresponding thereto; comparison means for comparing the DC levelsignal generated by said circuit means with a reference voltage togenerate an output when said DC level is lower than said referencevoltage; alarm signal generating means for generating an alarm outputwhen the output of said comparison means lasts over a predeterminedstorage time; and delay circuit means for supplying the DC level signalfrom said circuit means as said AGC control voltage to said automaticgain control amplifying means with predetermined delay time exceedingthe storage time of said alarm signal generating means, whereby anautomatic gain control is performed such that the input to saidcomparison means is maintained at a constant level against variations inthe received light signal level of said light sensitive means with apredetermined time delay within an automatic gain control range when thereceived light signal level of said light sensitive means is higher thana predetermined level.
 4. A photoelectric intrusion detectorcomprising:light sensitive means for receiving a pulsed light projectedfrom a light emitting unit at a place remote from said light emittingunit to generate a corresponding electric signal; circuit means forreceiving the electric signal generated by said light sensitive means togenerate an output signal having a DC level corresponding thereto;memory means for storing as an initial value a DC level signal generatedby said circuit means when said detector is connected to a power source;correction factor alteration means for comparing a value of the DC levelsignal generated by said circuit means with the initial value stored insaid memory means at predetermined intervals to generate a correctionfactor output varying in correspondence to a difference therebetween;operation means for performing an operation on the correction factoroutput from said correction factor alteration means and DC level signalgenerated by said circuit means to correct said DC level signal tofollow said stored initial value with a predetermined time delay;comparison means for comparing the corrected DC level signal generatedfrom said operator means with a predetermined reference voltage togenerate an output when the DC level of said corrected DC level signalis lower than said reference voltage; and alarm signal generating meansfor generating an alarm output when the output of said comparison meanslasts over a predetermined storage time shorter than said predeterminedinterval.